Mg(PF₆)₂‑Based Electrolyte Systems: Understanding Electrolyte–Electrode Interactions for the Development of Mg-Ion Batteries

Mg­(PF₆)₂-based electrolytes for Mg-ion batteries have not received the same attention as the analogous LiPF₆-based electrolytes used in most Li-ion cells owing to the perception that the PF₆^– anion decomposes on and passivates Mg electrodes. No synthesis of the Mg­(PF₆)₂ salt has been reported, nor have its solutions been studied electrochemically. Here, we report the synthesis of the complex Mg­(PF₆)₂­(CH₃CN)₆ and its solution-state electrochemistry. Solutions of Mg­(PF₆)₂­(CH₃CN)₆ in CH₃CN and CH₃CN/THF mixtures exhibit high conductivities (up to 28 mS·cm^–1) and electrochemical stability up to at least 4 V vs Mg on Al electrodes. Contrary to established perceptions, Mg electrodes are observed to remain electrochemically active when cycled in the presence of these Mg­(PF₆)₂-based electrolytes, with no fluoride (i.e., MgF₂) formed on the Mg surface. Stainless steel electrodes are found to corrode when cycled in the presence of Mg­(PF₆)₂ solutions, but Al electrodes are passivated. The electrolytes have been used in a prototype Mg battery with a Mg anode and Chevrel (Mo₃S₄)-phase cathode